1. Field of the Invention
The present invention relates to data communication systems, and more particularly to data communication systems in which a central modem (modulator-demodulator) communicates with a number of remotely located modems and data stations.
2. Background of the Invention
In data communication systems, it is frequently desirable to be able to communicate data to and from a number of remotely located points and a central location. For example, a number of date entry and/or display terminals can be connected via individual data communication channels (e.g. telephone lines) to a central processing unit, computer or other data terminal.
As is well known, because of various constraints digital data signals cannot be sent directly over band-limited telephone lines but rather must be changed into a form suitable for transmission. Whereby two-way transmission over such data communication channels is desired a device called a modulator-demodulator (modem) is used at each end of the line to convert data signals into a form suitable for transmission over a telephone line and to convert signals received over the telephone line back into a digital data stream suitable for processing by a computer or other data handling apparatus. Each modem thus includes a suitable data transmitter and receiver section.
As is well known, the electrical characteristics of telephone lines which are used for data transmission can vary according to the distance from the remote station to the central station, the time of day, and whether a special leased line is used or the central station is dialed up through the telephone system's direct distance dialing network. Especially in the latter case, the characteristics of each line can change upon each connection.
In order to overcome the adverse effects of variable line characteristics, it is known to use a device called an equalizer which, in effect, electrically counteracts or corrects the distortion caused by the line.
Equalizers fall into two broad catagories: fixed and adaptive. In a fixed equalizer, the average electrical characteristics of a telephone channel are determined and a fixed amount of equalization is then embodied in the equalizer to counteract any distorting characteristics of an average telephone channel. In an adaptive equalizer, the telephone channel is tested either upon its initial connection or between data transmission and corrections are made to the equalization characteristics (so-called equalizer coefficients) of the equalizer in order to counteract the detected distortions in the telephone channel.
Where there are a number of remote data generating/receiving stations, of necessity the central data processing apparatus must "poll" or look at each of the data communication channels in a predetermined sequence to enable the users of each of the remote stations to have access to the central station. If this polling is performed quickly enough the user of the remote station will not notice any appreciable lag in any response from the central station.
In prior art arrangements for polling modems, it has been the practice to provide each of the remote modems with a fixed equalizer and the central modem (connected to the data processing apparatus) with a plurality of equalizers associated with each of the plurality of transmission channels from the various remote polled modems. A training period is provided to adjust each of the central station equalizers to each channel. The appropriate equalizer at the central station is automatically switched for connection to the receive line of the central station when the polled remote station provides a response.
However, it will be appreciated that this arrangement requires a separate equalizer at the central modem for each data transmission channel and that the central station must know which channel it is polling at any given time so as to switch in the proper equalizer for that channel.